For instance, organic sludge generated in a sewage treating process for public sewage, industrial sewage, etc. contain combustible or flammable components and are effectively utilizable as fuel by removal of the contained moisture content. Therefore, there has recently been proposed recycling of such organic sludge into solid fuel. Examples of such techniques for producing solid fuels are disclosed in Patent Documents 1 to 3.
As the technique of making solid fuels from sludge by carbonization, Patent Document 1 discloses a method including obtaining sludge carbide by carbonization of sludge in a carbonizing furnace, cooling the sludge carbide through a water cooling conveyer, pneumatically delivering the cooled sludge by inert gas through a gas-flow conveying pipe, separating and collecting the sludge carbide contained in the conveying gas-flow by a bag filter, and then humidifying the sludge carbide by a humidifier to produce sludge carbide fuel. A technique of producing solid fuel from sewage sludge by the carbonization method as in Patent Document 1 is generally widely used and has been developed day by day for further technological advance. On the other hand, other techniques of producing solid fuel from organic sludge as in Patent Documents 2 and 3, different from the carbonization treatment method, have also been developed.
Patent Document 2 discloses a technique of producing odorless solid fuel, in factories or plants in which active sewage sludge are generated on site, in such a manner that active sludge is mixed with coffee waste having a deodorizing function and coagulated organic sludge by stirring to produce an active sludge mixture, and drying the mixture naturally at room temperature. In Patent Document 2, it takes two or three days in summer and about one week in winter to dry the active sludge mixture until a moisture content becomes about 20 to 25%. Even after stirring, drying for 5 minutes at a temperature of 80° C. in a high-temperature drying device, and then leaving at room temperature, the drying time needs one day.
In Patent Document 3, moreover, in a pre-treatment process, organic waste such as sewage sludge and food waste is pulverized and kneaded under pressure so that it is heated to 20 to 40° C. by frictional heat generated during kneading, thereby making the organic waste easy to ferment aerobic microbe existing in the organic waste. Thereafter, in a fermentation step, in a heat-insulated fermenting vessel, the fermented material is stirred to be broken up or subdivided, while the fermentation is prompted by air supply, thereby decreasing the moisture content of the fermented material. In a next breakup process, in a fermenter, the fermented material is scooped up by blades driven by a belt and dropped down to prompt further breakup and fermentation to ripen, thereby producing solid fuel. In Patent Document 3, a treatment time needed from the start of the pre-treatment process to the completion of producing the solid fuel takes at least 10 days.